Method and device for packaging audio samples of a non-PCM encoded audio bitstream into a sequence of frames

ABSTRACT

An audio bitstream is read from a digital video disc DVD for transfer, subsequent to parsing thereof, via an IEC 958 protocolled interface, for use in a multi-channel audio reproduction apparatus. For each respective audio channel MPEG audio samples are packaged recurrently in burst payloads, and these burst payloads are packaged as user data in IEC958 format frames. Pause bursts are used for signalling absence of audio for all associated channels with, each pause burst representing such audio absence during a perceptibly acceptable time interval only.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit or priority of and describes therelationships between the following applications: wherein thisapplication is an application for reissue of U.S. Pat. No. 6,070,062,which issued Jun. 13, 2000, and is also a continuation of U.S. patentapplication Ser. No. 10/170,822, filed Jun. 13, 2002, which is also anapplication for reissue of U.S. Pat. No. 6,076,062, from U.S. patentapplication Ser. No. 08/798,198, filed Dec. 9, 1996, which is theNational Stage of International Application No. PCT/IB1996/001267, filedNov. 21, 1996, which claims the priority of foreign application EPO95203376 filed Dec. 7, 1995.

BACKGROUND OF THE INVENTION

The invention relates to a method of transferring a non-PCM encodedaudio bitstream read from a digital medium, subsequent to parsingthereof, via an IEC 958 protocolled interface to a multi-channel audioreproduction apparatus.

Digital video disc standardizing is proceeding at an accelerated pace.Commercially available MPEG1 decoder circuit SAA2502 is able to decodecompressed digital audio received as a continuous bit stream.Present-day MPEG2 technology has standardized 5 channels, to wit: Left,Right, Center, Left Surround, Right Surround, and furthermore a lowfrequency enhancement (LFE) channel. The MPEG2 bit stream is distributedinto frames of 1152 samples for each of the actual channels, and playeroperation is controllable in a non-uniform manner on a frame-to-framebasis. For example, the number of actual channels may vary, and certainones or all of them may be outputting silence.

General background to the invention is given by the following earlierdocuments, all being at least co-assigned to the present assignee andbeing herein incorporated by reference:

EP Patent 402 973, EP Patent Application 660 540, corresponding U.S.Pat. No. 5,323,396, issued Jun. 21, 1994, which is a continuation ofU.S. application Ser. No. 07/532,462, abandoned; U.S. Pat. No.5,606,618, issued Feb. 25, 1997; U.S. Pat. No. 5,530,655, issued Jun.25, 1996; U.S. Pat. No. 5,539,829, issued Jul. 23, 1996; U.S. Pat. No.5,777,992, issued Jul. 7, 1998; and pending U.S. application Ser. No.08/488,536, describing a Musicam Layer 1 encoder and decoder for L and Rsignals;

EP 678 226, corresponding to U.S. Pat. No. 5,544,247, issued Aug. 6,1996, describing encoding and decoding of L, R and C channels;

U.S. patent application Ser. Nos. 08/032,915, 08/180,004, 08/427,046,describing the matrixing of bitrate-reduced L, R, C, SL and SR signals.

Now, in a consumer application (SPDIF) of the above specified digitalvideo disc, two subframes are specified that each can simultaneouslycarry 32 bit data words. This allows to transfer via the IEC 958bitstream either 2-channel linear PCM audio, or a set of alternatingbitstreams, but not those configurations simultaneously. The IEC 958standard specifies a widely used method for interconnecting digitalaudio equipment with 2-channel linear PCM audio. A need has beenencountered to allow transferring non-PcM encoded audio bitstreams forconsumer applications in the same protocolled environment, and inparticular pause bursts, in case one or more of the audio channels wouldrepresent silence. In particular, the granularity of such pauserepresentation at the receiver side should be sufficiently brief from aperceptive standpoint.

SUMMARY OF THE INVENTION

Accordingly, amongst other things, it is an object of the presentinvention to extend present protocols to allow transfer of non-PCMencoded audio bitstreams for consumer applications in the sameprotocolled environment.

This and other objects, features and advantages according to the presentinvention are provided by a method for decoding a non-PCM encoded audiobit stream, which can be read from a digital medium. Preferably, themethod includes the steps, executed for each respective audio channel,of recurrently packaging MPEG audio samples in burst payloads, andpackaging the burst payloads as user data in IEC958 format frames,including pause bursts which signal the absence of audio for allassociated channels. According to one aspect of the present invention,each pause burst represents such audio absence during a perceptivelyacceptable time interval only.

According to the state of the art, the above granularity could reachseveral tens of milliseconds, which the present inventor has foundunacceptably long. According to the invention, the granularity is in themillisecond range which is acceptable in all circumstances encounteredat present.

These and other objects, features and advantages according to thepresent invention are proved by a method for receiving a non-PCM encodedaudio bitstream emanating from a parsed bitstream read from a digitalversatile disc DVD via an IEC 958 protocolled interface for use in amulti-channel reproduction apparatus. Advantageously, the methodincludes steps for receiving the parsed bitstream as a sequence offrames each accommodating, for each applicable bitstream, a uniformnumber of data bits, storing each frame in an intermediate frame buffer;detecting presence or absence of data pertaining to a particular outputchannel. Moreover, in response to the detecting step, executing decodingand outputting decoded information for the particular channel. However,under control of one or more pause bursts received, as representing asequential multiplicity of absence of the detecting, controlling a softmute block. It will be appreciated that this feature provides forstraightforward decoding of the pause bursts for subsequentrepresentation by a soft mute block.

These and other objects, features and advantages according to thepresent invention are provided by a device for implementing the methoddescribed immediately above, either on the encoding or the decodingside. Further advantageous aspects of the invention are set forthhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further aspects and advantages of the invention will bediscussed more in detail hereinafter with reference to the disclosure ofpreferred embodiments, and more in particular with reference to thefollowing drawings, in which:

FIGS. 1-3, 4A, 4B, 4C, 4D and 5, show various information formats;

FIG. 6, a block diagram of connected DVD player and MC_Box;

FIG. 7, a block diagram of a multi-channel audio decoder;

FIG. 8, a decoding flow chart of a digital signal processor;

FIG. 9, a decoding flow chart of a subband filter DSP;

FIG. 10, a block diagram of an IEC 958 transmitter station;

FIG. 11, a block diagram of an IEC 958 receiver station;

FIG. 12, a flow chart of bit stream transmission.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. Data Formats

For better detailing the invention, first various applicable informationformats are described. FIG. 1 shows the IEC958 format, which accordingto the upper diagram consists of a concatenation of frames grouped intoblocks of 192 frames each. The second diagram shows each frame toconsist of two sub-frames. The first frame of a block has subframeslabelled B (left) and W (right), further subframes are all labelled M.The third diagram shows the setup of a subframe. As shown, it has fourpreamble bits, four auxiliary bits, four unused bits, sixteen data bitsor bitstream, and four flag bits V, U, C, P. The flag bits indicate thefollowing:

-   -   V indicates no deviation from the standard;    -   U indicates user data with ‘0’ default;    -   C contains one bit of a channel status word; and    -   P is a parity bit relating to bits 4 through 31.        A pair of subframes may contain one PCM word from each of left        and right channels.

According to the present invention, for consumer applications, thechannel status word as built from a sequence of C bits has the followingmeaning: bit bO with value 0 indicates consumer PCM audio, bit bi withvalue 1 indicates Non-linear PCM samples, bits 8 through 15 contain acategory code. Furthermore, the MPEG header indicates audio sample rateand sample size in bits.

Now, audio bit streams as read from the DVD disc may contain gaps, thatmay be due to pauses in the audio, or to a trick mode of the relatedvideo source, such as the transition to a freeze picture produced intrack mode. Now, during transfer in bursts on the IEC 958, these gaps inthe bit stream may remain unused or rather, filled with bursts of thedata_type ‘pause’ to be described hereinafter. If the gap occurs inMPEG1 layer 1, or in MPEG 1 layer 2 or 3 data, or in MPEG2 withoutextension, or in MPEG2 data with extension audio bitstream, the gap willbe filled with a sequence of bursts of data_type ‘pause’. These burstsmay have the minimum allowable length therefor, corresponding to 32sample periods. Preferably, this number is three times as long, socorresponding to 96 sample periods, which is the recurrency of LFEsamples.

It is possible on this interface to simultaneously convey multiplemulti-channel non_PCM encoded data streams, for example relating to botha main audio service and to an associated audio service. In that case,the burst of the associated service occurs before the burst of the mainservice to which it is associated.

FIG. 2 describes the burst format attained by unpacking the userinformation of an IEC 958 block in a standard manner, or rather beforethe packing of an IEC 958 block. The burst has a fixed repetition timerelated to the number of audio samples for each channel encoded withinthat frame. Any unused bit between two bursts is set to zero. Each bursthas four sixteen-bit preamble words, with the following meanings: Pa, Pbsync words, Pc burst info to be specified hereinafter, Pd length ofpayload in bits. Subsequently, the burst contains a payload field and isoptionally terminated by stuffing zeroes to attain its pre-specifiedformat. An advantageous, but not mandatory lower bound for the number ofstuffing zeroes is 32. The payload also contains the MPEG header. Theformat of non-PCM encoded Audio bitstreams allows multiplexed conveyingof more than one bitstream, wherein a burst can fill the space ofstuffed zeroes from other bursts. The sampling frequency must be uniformacross the bursts. The field Pc has the following codes:

Pc repetition of burst in Bits value content number of sample periods0-4 0 Null data @4096  1 AC-3 stream 1536 2 SMPTE time stamp 3 MPEG1layer 1 data  384 4 MPEG1 layer ⅔ or MPEG 2 1152 without extension 5MPEG2 with extension 1152 6 PAUSE 32 or 96 7 ACX data 1024 8 MPEG 2layer 1 low sample rate  384 9 MPEG2 layer ⅔ low rate 1152 10-31ReservedThe content of further bits of Pc is irrelevant to the presentinvention. The provision of the relatively brief ‘pause’ burst allows alow granularity size of ‘soft mute’ intervals controlled thereby. Theindication of the various burst type specifications by Pc bit values 3,4, 5, 8, 9, allows an extremely flexible control policy.

FIG. 3 shows an MPEG1 layer 1 base frame that has a length of 384 sampleperiods (each of L and R). The various aspects of the format have beenconsidered supra. The base frame for the payload of the MPEG1 layer ⅔ orMPEG2 without extension has the same shape, be it with a length of 1152instead of 384 sample periods. MPEG2 allows transfer of five audiochannels in parallel. In certain circumstances, the MPEG2 burst needs anextension that has been shown in FIGS. 4B and 4C.

Now, an MPEG2 frame comprises 1152 samples for each encoded channel. Theburst as shown in the uppermost row, is headed by a burst_preamble,followed by the payload, and stuffed with stuffing zero bits. Thepayload numbers up to 36768=1152×32 bits. Furthermore, there are atleast 32 stuffing zeroes and 64 bits for the Pa..Pd header. Bitstreamsmatching the MPEG layer 2 data type are:

-   -   either encoded according to MPEG2 layer 2 or 3,    -   or even encoded according to MPEG2 layer 1 ‘superframe’.

A burst with an Audio frame consists of a synchronized and concatenatedBase frame (MPEG1 compatible) and an extension frame. FIG. 4A shows theMPEG2 layer 2 base frame, with MPEG1 header, MPEGi Audio field, MC(multichannel) extension part i field, and a field for ancillary data.If extension is necessary, additional format according to FIG. 4B isappended that contains in MPEG2 layer 2 Extension frame an Extensionheader followed by the MC extension part 2. FIG. 4C of this shows theformats of FIGS. 4A and 4B both synchronized and concatenated. Likewise,the FIG. 4D shows the MPEG2 base frame, with extension frame asfractional payload inside the burst repetition time represented by thelower arrow, that in its turn needs a preamble and allows for trailedstuffing zeroes to the frame.

Now, by using units of 32 sample periods per subband filter,synchronization is maintained. In this respect, FIG. 5 shows a burst ofdata type ‘PAUSE’ inside its burst repetition time indicated by thebottom arrow. The length is 1024 bits=32 IEC frames, increased with thestuffing zeroes. As earlier, the four indications Pa, Pb, Pc, Pd arepreappended. The user content is all zero. Another and preferred size ofthe burst allows for 3×32=96 frames. The burst frame has of course adummy content; the longer version allows better synchronization to theLFE feature that occurs every 96 frames. Due to the relatively smallsize of pause bursts, the transition between pause and non-pause has asmall granularity size.

2. Hardware Embodiments

FIG. 6 is a block diagram of an interconnected DVD player 30 and anMCBox 46. In player 30, block 20 symbolizes a turn-table and associatedread-out and feedback mechanisms, associated control signals beingtransferred by means of control path 21. Control processing is inmicroprocessor 26. Block 22 is an MPEG2 program stream decoder and audioparser that separates the massive bitstream received into standardstereo audio and video streams on the one hand to go to audio-videodecoder 24, and furthermore, multi-channel bitrate reduced audio data onchannel 23. Audio-Video decoder 24 operates in a standard manner forseparating the bit stream into left and right audio channels and videoas indicated. In fact, this type of reproduction is conform to the MPEG1standardization. Relatively low-level consumer applications would dowith the system as described thus far. DVD player 30 is implemented witha user control interface, such as hard buttons, soft keys, display.

For attaining full functionality of MPEG2, an external multichannelMC_box 46 has been provided. To this effect, first in player 30, theMPEG data is configured according to the burst format described withrespect to the earlier Figures. Next, this requires an output channel 33for data according to the standardized IEC 958 protocol, and which isused to convey a non-PCM bitstream inclusive of various commands for theMC_box. The channel may be based on galvanic interconnection or opticalfibre. Optionally, interconnection is by a uni- or bidirectional channel48, in particular for transmitting commands to the DVD player. Thechannel may be protocolled according to D2B described in U.S. Pat. No.4,429,384 to the present assignee. Moreover as shown, a FIFO 28 isprovided that by way of example accommodates 8 k Bytes as generallyrequired for intermediate storage of MPEG data, a bus interface circuit32 of commercially available type TDA1315, and a control interfacecircuit 34 of type MSM6307, organized according to the D2B protocol.Alternatively, block 32 receives commands from the microprocessor 26 onthe data path, rather than on its control path.

Like the DVD player, MC_box 46 has an internal control path 41,interface circuit 38 of type MSM6307, and control processing inmicroprocessor 40. In correspondence to FIFO 28, the MC_box 46 has arelatively small FIFO 44. This stores the data of one bitstream whilethe previous one is decoded locally. The decoding pertains first to theburst level, and next to the sample level. The output from FIFO 44 feedsMC_decoder 42 that may output up to seven audio channels as indicated:Left, Right, LFE/C, Left center surround, Right center surround, Leftsurround and Right surround. As shown, these are grouped on four I2Sinterfaces, according to a protocol described in U.S. Pat. No. 4,755,817to the present assignee. Alternatively FIFO 44 plus decoder 42 arecombined into a single hardware block and controlled directly by thecommands contained in the IEC 958 data. Moreover, the MC Box attaches tothe secondary control channel 48 by means of circuit 38.

FIG. 7 is a block diagram of a multi-channel audio decoder, as containedin block 42 in FIG. 11. First, the decoding proper is executed in block56 according to the process described with reference to FIG. 8, andimplemented with a Motorola DSP processor of the 56000 seriesarchitecture. Also, the dematrixing is executed in this processor. Block54 symbolizes a control shell to the processor in question. The outputof the first DSP processor is organized in blocks that each contain foreach appropriate channel 3*32=96 subsamples. For such channel at thehighest applicable sample frequency of 44.1 kHz, the block lengthcorresponds to an interval of 2 msec, which is considered a sufficientlyfine granularity to be practically imperceptible.

Block 58 is an intermediate buffer that can hold n blocks as specifiedsupra, optimized as regards to cost versus allowable occurrence ofover/underflow; expected value of n is about four. Line 70 transmits astop/go signal to DSP shell 54 that functions as source; line 68transmits a request signal from the data destination block 60.

Block 60 executes the demultiplexing function with respect to themaximum of seven channels received; it is based on a similar Motorola56000 DSP processor. In particular, block 62 symbolizes the subbandfiltering, whereas block 64 symbolizes an LFE ups ample filter. Again,the processor shell has been indicated by block 66. During eachexecution cycle, 32 subchannels per channel are filtered, and unloadedby means of a dual port RAM: the length of a cycle is thus for a samplefrequency of 44.1 kHz:32/44, lk=0.725 millisec. The delay length of theRAM for each channel is advantageously equal to 3*32 subsamples.Filtering takes place when 3*32 subsamples have been received, otherwisethe subband filter will output all zeroes signalling an audio pause,which thus has a reduced granularity with respect to prior art. Theprocessor will contain a ‘free running’ function, and will continuallyoutput audio samples at uniform intervals. The first DSP 56 willcontinually produce audio samples in bursts of 1152 samples per channel,12 groups of 3*32 samples each for each and every channel. The real-timedemand is on subband filter 62. If applicable, decoder 56/54 is put on“hold” to avoid an overflow of buffer 58.

The MC Box does not have a user control interface, but the data receivedon the IEC unidirectional interconnect 33 are used for effectingcontrol, inclusive of the soft-mute and concealing feature according tothe invention. If required, the D2B interconnect allows for sendingcontrol signals in the reverse direction. The multichannel decoder 60can be controlled by decoder 54, such as by means of an I2C interface asdisclosed in U.S. Pat. No. 4,689,740. This will be robust enough torecover from error conditions. However, no status output to a user isdeemed necessary. If underflow occurs in buffer 58, the soft mutefeature is controlled subsequently.

FIG. 8 shows a decoding flow chart of a processor, in particular digitalsignal processor 54 in FIG. 7. The received input bitstream issymbolized by 74, on which the decoder continually undertakes asynchronizing operation 76. Actual decoding starts in block 78 as soonas DSP 54 synchronizes and a next concatenated frame through thesynchronizing words Pa, Pb is received. Frame item Pd gives the lengthof the payload. When synchronized, decoder 54 for each frame producestwelve groups of 3*32 sub-samples for each channel. Decoder 56 is put onhold when the free area in the sub-sample buffer is not sufficient tostore all sub-samples of that group (of 3*32) sub-samples for eachchannel to avoid buffer overflow.

The hand-shake between the sub-sample buffer and decoder DSP 56 isimplemented by a token that indicates the current owner of the block inquestion; this token is transferred when synchronization has beeneffected (77, 88). In the flowchart, block 78 detects either audio data,or a pause. Unless third pause, data type detecting is continued (78).Upon meeting a non-pause, decoding is continued in block 80, and thedecoding result is outputted on line 81, subject to the reception of ablocking token to put the processor on hold via block 82. The handshakeis between blocks 80 and 82. Bidirectional connection 83 allows reactingto the filling degree of buffer 58. When the third pause is received(84), block 86 prepares zero output blocks for outputting on line 81 asan alternative to the decoding results from block 80, to function as‘soft mute’ information.

FIG. 8 is a decoding flow chart of subband DSP filter 62 in FIG. 7. Eachcycle, the subband DSP receives 32 subsamples per channel at its input;if no subsamples are available, the input will become all zeroes as asoft-mute. The subband filter DSP processes blocks of 32 sub-samples andproduces 7 out of 8 signals for the four I2S interfaces shown in theFigure. The eighth signal LFE will be upsampled by block 64. The filteroperates according to a continuous process, producing audio atequidistant time intervals. After power-on, all outputs are muted bydefault; the output registers will contain zeroes until the subbandfilter is initialised after 512 sample periods.

In the implementation, block 50 detects whether the buffer 58 is notempty. If empty, zeroes are output in such a way as to maintainsynchronization. If not empty and a token has been passed, the blockoutput is at right, and 32 samples are outputted for each actualchannel, plus a single LFE sample. If not empty and no token has beenpassed, the block output is at left, and 32 zero subband samples areoutputted to emulate a pause. Both outputs from block 50 lead to theinput of subb and filter 62 and LFE upsample filter 64 in FIG. 7.

The token indicates which processor is currently the owner of the block.An owner has read/write access to a block, non-owners can only read,such as read the token. Block ownership is only passed along by theowner of the block, render the actual owner to non-owner. Afterpower-on, all tokens will be handed to the decoder DSP. Absent a token,the subband filter will clear all registers and will filter exclusivelyzeroes. When synchronizing on the Burst_Preamble, the first token shallbe passed to the subband filter DSP after an expected ‘worst case’decoding time.

FIG. 10 is a block diagram of an IEC 958 transmitter station, of which acentral part is the commercially available TDA 1315 circuit 98interconnected as shown. Block 90 symbolizes the parser of synchronizedand concatenated Base and Extension bitstreams (block 22 in FIG. 11).Microprocessor 92 corresponds to microprocessor 26 in FIG. 11.Microprocessor 92 interacts with interface circuit 98 along a three-wireL3 control bus protocolled according to U.S. Pat. No. 5,434,862 assignedto the present assignee, and connected to pins 23, 24, 25 as shown. Thedata output from block 90 is protocolled according to the I2S format andconnected on pins 35, 36, 37 as shown. Input 32 receives a mute controlsignal from parser 90, pin 33 an I2S selection signal and pin 38 an I2Soutput enable signal, these two continuously at logic 1. Timing controlblock 96 is controlled by microprocessor 92, and handshakes alonginterconnection 93 with parser block 90. Also, it handshakes on a synccycle basis with TDA 1315 on pins 39, 40. Finally, the circuit outputsserial data according to IEC 958 on pin 8, and receives an enable signalon pin 9 at a continuously low value. Block 100 is an electric tooptical converter, allowing a remote position of the MC_Box.

FIG. 11 is a block diagram of an IEC 958 receiver station. Data isreceived as 16 bit words over optical-to-electric converter 102 andtransferred to IEC 958 input pin 6. Standard control pins are ConTRLMODEat pin 21, IECSELection pin 7, and I2SOoutputENable pin 38, all threeheld at logic ground. Furthermore, there are IECOEoutputenable pin 9 andCLIocK SELection pin 43, both held at logic high (1). The clockselection allows to select between 384 kHz and 256 kHz. The data outputfrom TDA 1315 is by means of an I2S protocol on pins 35, 36, 37 tomulti-channel decoder 108. This produces the four output bit streams asdeemed in FIG. 11. Control interconnection between TDA 1315, themicroprocessor (item 40 in FIG. 11) and multi-channel decoder 108 isprotocolled according to the 12S protocol referred supra.

FIG. 12 is a flow chart of bitstream transmission. In block 120,transmission is started. Channel status bit 1 becomes “1” In block 122IEC 958 “Idle” is detected. If “Idle”, in block 124 it is detectedwhether NULL data is needed. If “No”, the system reverts to block 122.If “Yes”, in block 132 a NULL data burst is sent; the latter feature isoptional. If in block 122 an Audio bitstream is detected, in block 126it is detected whether a Gap occurs. If “Gap”, in block 120 a PAUSE databurst is sent. Also, the repetition time is set. If in block 126, anAudio data-burst is detected, an audio data burst is sent. Also, therepetition time is set. Both after block 128 and after block 130, inblock 134 it is detected whether the repetition time has finished. IfNo, in block 136 “stuffing” is executed, and the system reverts to block134. If in block 134, the repetition time has finished, the systemreverts to block 126.

PAUSE data-bursts are intended to fill small discontinuities in thebitstream, the gaps which may occur between two data-bursts of a non-PCMencoded audio data type. PAUSE data-bursts convey information of theaudio decoder that a gap exists. The PAUSE data-bursts may also indicatethe actual length of the audio gap, or that the non-PCM audio datastream has stopped. This information may be used by the audio decoder tominimise (or conceal) the existence of the audio gap, or in the casethat the bitstream stops, to trigger a fade-out of the audio. A sequenceof PAUSE data-bursts can also assist decoder synchronization prior tothe beginning of a non-PCM audio bitstream. It is recommended to send ashort sequence of PAUSE data-bursts immediately preceding thetransmission of the first audio data-burst.

Data stuff- P stuff- P Stuff- P stuff- Data stuff- Data burst ing inging ing burst ing burst R R R R R R

In this example, P indicates a PAUSE burst, P+subsequent stuffingrepresents the repetition time of PAUSE, and the total gap in betweenthe data bursts is three times as long. The length of Databurst+stuffing is the repetition time of the burst. The PAUSE burst istransferred with the same bit stream number as the bit stream number ofthe audio data stream which contains the gap that the PAUSE data-burstsare filling, or for which synchronization is being assisted.

The PAUSE data-burst contains the burst_preamble and a 32-bit payload.The first 16 bits of the payload contain the audio gap_length parameter.The remaining bis are reserved, and should all be set to ‘0’. The audiogap_length parameter is an optional indication of the actual audio gaplength. This is the length, measured in IEC958 frames, between the firstbit of Pa of the first PAUSE data-burst and the first bit of Pa of thenext Audio data-burst. The detailed use of the PAUSE data-burst dependson the data-type of the Audio data-burst. For example, gaps between AC-3data bursts are filled with a sequence of very short PAUSE bursts, andthe repetition time of PAUSE data-bursts between data-bursts of an MPEGtype is related to the algorithm. The gap_length parameter of the firstPAUSE data-burst of the sequence may be used to indicate the length ofthe audio gap which is being bridged by the sequence of PAUSEdata-bursts. The PAUSE data-bursts in the sequence which follows theinitial PAUSE data-burst do not have a gap_length specified(gap_length=0). A gap may be filled with one single sequence of PAUSEdata-bursts with a single indication of audio gap_length. For example, agap corresponding to an audio gap of 768 samples may be filled with onesequence of PAUSE data-bursts with an indication of gap-length=768 inthe initial PAUSE data-burst. Or this gap could be filled with a numberof smaller sequences of PAUSE data-bursts, with the initial PAUSEdata-burst in each sequence indicating the gap_length bridged by thatsequence (E.g. one sequence with a gap-length of 200 samples, followedby a sequence with gap-length of 568, together bridging a gap of 768sample periods).

The information about the full length of the audio gap in the firstPAUSE data-burst will allow the decoder to perform the best concealment.However, if the data source does not have the information about the fullaudio gap length at the time the gap begins, then it may signal aninitial value for gap_length. If the data source then determines thatthe audio gap will be longer than the initial indication, then anothersequence of PAUSE data-bursts may be initiated (following the firstsequence by the repetition time) with another gap_length value to signalthe decoder that the audio gap is being extended. If the gap is furtherextended, additional sequences may be initiated.

Audio decoders may use the gap_length information to optimise theirconcealment of the audio gap. The inclusion of non-zero values ofgap_length is optional, data sources are not required to indicate thelength of the audio gap.

The data type PAUSE has a sequence of four control words Pa, Pb, Pc, Pdfollowed by the Payload and the Stuffing.

“Gaps” are discontinuities in a bitstreams, and may be due to switchingbetween bitstreams. The length of the gaps depend on the timing ofswitching from one to the other bitstream, and may have any value. Thelength of a gap however, depends on the decoder which must be able toconceal the gaps. Therefore the transmitter shall adjust the length ofthe gap to a multiple of the repetition time of PAUSE data-bursts. ThePAUSE data-burst has its repetition time, which gives the time oftransmission of Pa of the next data-burst.

Some AC-3 decoders may be capable of “concealing” audio gaps. Theindication of the audio gap length (gap_length) which may be included inthe payload of the PAUSE data-burst allows the decoder to know how longan audio gap will need to be concealed, and thus allow the decoder tooptimise the concealment process for the actual audio gap length. AC-3decoders will most easily conceal audio gaps which have a length equalto an integral multiple of 256 samples. Thus audio gaps of length 256,?68, etc. IC958 frames are preferred, as follows:

repetition time of PAUSE data-burst data-type of Audio data-burstmandatoty recommended AC-3 data 3 IEC958 frames MPEG-1 Layer 1 data 32IEC958 frames MPEG-1 Layer 2 or 3 data or 32 IEC958 frames MPEG-2without extension MPEG-2 data with extension 32 IEC958 frames MPEG-2,layer-1 Low sample rate 64 IEC958 frames MPEG-2, layer-2 or 3 Low sample64 IEC958 frames rate

The AC-3 bitstream consists of a sequence of AC-3-frames. The data-typeof a AC 3 data-burst is 01 h. An AC-3 frame contains 1536 samples foreach encoded channel. The data-burst is headed with a burst_preamble,followed by the burst_payload. The burst-payload of each data-burst ofAC-3 data shall contain one complete AC-3-frame. The length of theAC-3-data-burst will depend on the encoded bit rate (which determinesthe AC-3-frame length). An AC-3 data burst with reference instant Rcomprises again four control words Pa, Pb, Pc, Pd an AC-3 burst_payload,and stuffing.

I claim:
 1. A method of transferring a non-PCM encoded multichannelbitstream to digital audio reproduction apparatus via an interface whichis adapted for transfer of PCM encoded bitstreams, the non-PCM encodedmultichannel bitstream resulting from parsing a multichannel bitstreamreceived from a digital video disc (DVD), comprising the steps of: (i)recurrently packaging audio samples of the non-PCM encoded multichannelbitstream in data burst payloads; (ii) packing the data burst payloadsas user data in successive frames having a predetermined formatincluding pause bursts signifying the absence of audio in all channels,each pause burst having a duration in the range below 10 milliseconds;and (iii) decoding and supplying the packaged data burst payloads of therespective channels to respective channel inputs of said audioreproduction apparatus.
 2. A method as claimed in claim 1, wherein eachpause burst has a repetition interval of 32 or a multiple of 32 of theperiods of said audio samples.
 3. A method as claimed in claim 1,wherein the presence of a pause burst is indicated by a burst header inthe non-PCM encoded multichannel bitstream.
 4. A method as claimed inclaim 1, wherein each pause burst has a repetition period not overone-fifth that of any non-pause burst in the non-PCM encodedmultichannel bitstream.
 5. A method as claimed in claim 1, wherein thedata bursts have a repetition period of 384 periods of the audiosamples.
 6. A method as claimed in claim 1, further comprising addingstuffing zeros to the data burst payloads in order to maintain a uniformsize burst format, the bursts being multiplexed among the multiplechannels in a recurrent manner, relevant data from other channels beingsuperposed on stuffing zero positions in channels having stuffing zeros.7. A coupling device for transferring a non-PCM encoded multichannelbitstream to digital audio reproduction apparatus via an interface whichis adapted for transfer of PCM encoded bitstreams, the non-PCM encodedmultichannel bitstream resulting from parsing a multichannel bitstreamread from a digital video disc (DVD); comprising: means for recurrentlypackaging audio samples of the non-PCM encoded multichannel bitstream indata burst payloads; means for packing the data burst payloads as userdata in successive frames having a predetermined format including pausebursts signifying the absence of audio in all channels, each pause bursthaving a duration in the millisecond range below 10 milliseconds; andmeans for decoding and supplying the packaged data burst payloads of therespective channels to respective channel inputs of said audioreproduction apparatus.
 8. A method of transferring a non-PCM encodedmultichannel bitstream to digital audio reproduction apparatus via aninterface which is adapted for transfer of PCM encoded bitstreams, thenon-PCM encoded multichannel bitstream resulting from parsing amultichannel bitstream read from a digital video disc (DVD), the parsedbitstream being received as a sequence of frames which accumulates, foreach channel, a uniform number of data bits, comprising the steps of:storing each frame in an intermediate frame buffer; detecting thepresence of data pertaining to a particular output channel or theabsence of such data as represented by one or more sequential pausebursts; upon detecting the presence of said data in said detecting step,decoding and outputting decoded information for the particular channel;and upon detecting one or more sequential pause bursts in the detectingstep, controlling a soft mute block to conceal the absence of said data.9. A coupling device for transferring a non-PCM encoded multichannelbitstream to digital audio reproduction apparatus via an interface whichis adapted for transfer of PCM encoded bitstreams, the non-PCM encodedmultichannel bitstream resulting from parsing a multichannel bitstreamread from a digital video disc (DVD), comprising: means for receivingsaid parsed bitstream as a sequence of frames which each accommodate,for each channel, a uniform number of data bits; an intermediate framebuffer for storing each frame; detection means for detecting thepresence of data pertaining to a particular output channel or theabsence of such data as represented by one or more sequential pausebursts; decoding means controlled by said detection means for decodingand outputting from a stored frame decoded information for saidparticular channel; and means responsive to detection by said detectionmeans of said one or more sequential pause bursts to control an audiosoft mute block of said one or more sequential pause bursts to concealthe absence of data in said mute block.
 10. A device for packaging audiosamples of a non-PCM encoded multichannel audio bitstream into asequence of IEC958 format frames, the device comprising: a processorconfigured to detect the presence or absence of audio data in an audiochannel of a non-PCM encoded bitstream; generate pause data-bursts inresponse to the absence of audio data in the audio channel, recurrentlypackage data words of the audio channel of the non-PCM encoded audiobitstream into payloads of audio data-bursts when audio data is presentrecurrently package the audio data-bursts and the pause data-bursts in asequence of IEC958 format frames; output the sequence of IEC958 formatframes including the inserted pause data-bursts.
 11. The device of claim10, wherein the processor is configured to generate the data-bursts witha burst preamble and a payload, a size of the payload depending on thetype of the data-burst.
 12. The device of claim 11, wherein the burstpreamble consists of four 16-bit control words.
 13. The device of claim12, wherein information in a third control word of the four controlwords indicates the type of data-burst.
 14. The device of claim 10,wherein the audio samples are MPEG audio samples, and the processor isconfigured to generate the pause data-bursts with a repetition timeinterval that is equal to at most one MPEG LFE sample recurrency timeinterval, the pause data-burst repetition time being 96 audio sampleperiods.
 15. The device of claim 14, wherein the processor is configuredto generate the pause data-bursts with a repetition time interval equalto an integer multiple of 32 audio sample periods.
 16. The device ofclaim 10, wherein the processor is configured to indicate a presence ofthe pause data-bursts in a third control word in a header of the pausedata-bursts and to insert a 16 bit gap-length parameter in a payload ofa first pause data-burst of a series of pause data-bursts, and a 32-bitpayload of each pause burst being stuffed with zeros except the 16-bitgap length parameter.
 17. The device of claim 10, wherein the pausedata-bursts have a repetition time that is at least five times shorterthan a repetition time of any non-pause data-burst.
 18. The device ofclaim 10, wherein the processor is configured to generate MPEG-1 layerdata-bursts with a repetition time interval of 384 audio sample periodsper channel.
 19. The device of claim 10, wherein the processor isconfigured to generate MPEG-1 layer 2 or 3, or MPEG-2 without extensiontype data-bursts with a repetition time interval of 1152 audio sampleperiods per channel.
 20. The device of claim 10, wherein the processoris configured to generate MPEG-2 with extension type data-bursts with arepetition time interval of 1152 audio sample periods per channel. 21.The device of claim 10, wherein the processor is configured to generateMPEG-2 layer 1 low sample rate type data-bursts with a repetition timeinterval of 384 audio sample periods per channel.
 22. The device ofclaim 10, wherein the processor is configured to generate MPEG2 layer 2or 3 low sample rate type data-bursts with a repetition time interval of1152 audio sample periods per channel.
 23. The device of claim 10,wherein the processor is configured to add stuffing zeroes after thedate-bursts to fill a space between data-bursts, the minimum number ofstuffing zeros being 32, the amount of stuffing depending on apredetermined recurrency period for the date-bursts that depends on thetype of data-burst.
 24. The device of claim 10, wherein the audiochannel is one of a plurality of audio channels and the processorconfigured to recurrently multiplex the data-bursts among the pluralityof channels.
 25. A device for transferring a non-PCM encodedmultichannel audio bitstream, the device comprising: a processorconfigured to recurrently package audio data words of the non-PCMencoded multichannel bitstream into burst payloads of audio data-burststhat have a predetermined format, generate pause data-bursts indicatingan absence of audio samples in the bitstream for a predeterminedduration; and output successive data-bursts including the audiodata-bursts and the pause data-bursts.
 26. A device for transmitting anon-PCM encoded multichannel bitstream of audio samples with soft muteblocks to conceal the effect of audio data discontinuities when theaudio samples are played, the device comprising: a processor configuredto: detect the presence of an audio data discontinuity in a particularaudio channel in a bitstream; determine the length of the audio datadiscontinuity; generate one or more soft mute blocks depending on thelength of the audio data discontinuity; insert the soft mute blocks intothe audio bitstream to conceal the audio discontinuity when the audio isplayed; transmitting the audio bitstream containing the soft mute blocksto user audio equipment.
 27. A device for receiving a non-PCM encodedmultichannel bitstream of audio samples which have audio datadiscontinuities and for providing soft mute blocks to conceal the audiodata discontinuities when the audio samples are played, the devicecomprising: a processor configured to: receive a non-PCM encodedmultichannel bitstream of audio samples from user audio equipment;detect the presence of an audio data discontinuity in a particular audiochannel in a bitstream; determine the length of the audio datadiscontinuity; generate one or more soft mute blocks based on a lengthof the audio data discontinuity; insert the soft mute blocks into theaudio bitstream to conceal the audio discontinuity when the audio isplayed; output the audio bitstream including the soft mute blocks.
 28. Adevice for outputting audio samples of a non-PCM encoded multichannelaudio bitstream which have audio data discontinuities, the devicecomprising: a processor configured to detect the presence or absence ofaudio data in an audio channel of a non-PCM encoded bitstream;recurrently package data words of the audio channel of the non-PCMencoded audio bitstream into payloads of audio data-bursts when audiodata is present and into pause data-bursts in response to the audio datadiscontinuities in the audio channel; output the sequence of audiodata-bursts and pause data-bursts.
 29. A device for receiving a non-PCMencoded bitstream and outputting multichannel audio data for subsequentreproduction, the device comprising: a receiver configured to receive anon-PCM encoded bitstream for a plurality of audio channels as asequence of data bits; a decoder configured to detect a presence orabsence of audio data in the sequence of data bits and configured todecode audio information and output decoded audio information dependingon the detection of audio data and to conceal the absence of audio datadepending on the detection of the absence of audio data.
 30. The deviceof claim 29, wherein the absence of audio is detected by detectingpresence of one or more pause frames.
 31. The device of claim 29,wherein the decoder mutes the audio output based on the presence of oneor more pause frames.
 32. The device of claim 31, wherein the mute is asoft mute.